The traditional dyeing method of cotton fiber uses water as a medium, and the cotton fiber is dyed by adding a large amount of dye, salt and auxiliaries during the dyeing process. This method not only causes a dilemma of fighting for water between the printing-dyeing industry and the human beings and animals, but also causes serious environmental pollution due to inactivated dye, salt and auxiliaries contained in dyeing wastewater, which restricts the sustainable development of the textile printing and dyeing industry. Supercritical carbon dioxide anhydrous dyeing technology refers to a technology of dyeing fiber in a supercritical carbon dioxide medium, which has the advantages of nontoxic, tastelessness, low recovery cost of dyeing medium, etc., and can solve the wastewater problem in dyeing industry from the source.
At present, the supercritical carbon dioxide dyeing technology has enabled the synthetic fiber to have a satisfactory dyeing effect, and has gradually entered the stage of pilot test and trial production. However, the supercritical carbon dioxide dyeing technology for cotton fiber has not been broken through, which is due to the fact that the dye for cotton fiber contains a mass of sulfonic acid groups, which are hardly soluble in supercritical carbon dioxide. Natural dye has high safety factor, biodegradability, environmental protection and good affinity for natural fiber, and has been applied to dyeing natural fibers such as cotton, linen and silk thousands of years ago. More importantly, the natural dye does not contain the sulfonic acid groups in the structure and has good compatibility with the supercritical carbon dioxide medium.
According to literature reports, many natural dyes, such as turmeric, madder, lithospermum, rhubarb, etc., have certain solubility in supercritical carbon dioxide, have the basic conditions for dyeing cotton fiber in the supercritical carbon dioxide medium. However, the force between natural dye and cotton fiber is weak, and the problems of low color depth and poor color fastness exist after dyeing cotton fiber, which limits the application of the natural dye in the technology for dying cotton fiber in supercritical carbon dioxide.